This invention relates to improvements in gas discharge tube line protector modules of the type wherein a gas tube element in the line protector module is held in place by a spring which has been improved to optimize the performance of the module with respect to impact testing requirements while maintaining desired short-circuit and open circuit characteristics.
While the spring portion of the invention may be useful in a variety of applications, the ensuing description will be facilitated by specific reference to gas discharge tube line protection modules, and the problems of providing biasing means which will retain the desired compressive characteristics even after impact testing
Many forms of line protector have been developed which protect wire conductors and equipment connected thereto from electrical overvoltage and/or overcurrent conditions which may result from lightening, electrical power faults, and the like. Typically, these line protectors rely on a gas tube arrester which has an arc gap across which an overvoltage is be applied. The gas tubes of the line protector module are held within a retaining frame between biasing means on the one end and a solder pellet at the other. Overvoltages of short duration will cause conduction across the arc gap to ground. After the short duration overvoltage condition is past, the protector returns to its normal nonconducting state. Under long duration overvoltage (i.e., overcurrent) periods, the solder pellet is melted, whereby the gas tube is urged against the holding bracket to create a short condition through the circuit, thereby protecting the equipment or conducting lines attached thereto. These line protectors are designed to be modular so that they can be easily replaced once they have served their purpose in protecting the lines and/or equipment.
In most prior art systems the biasing means typically used to urge the gas tube against the pellet and bracket were springs, more specifically, coiled wire springs or helically coiled metallic strips known as volute springs. Such prior art springs which were used to hold gas tubes in compression achieved their primary purpose, however the prior art springs capable of surviving standard drop test requirements under various quality control and certification procedures were very expensive. Two types of testing procedures are conducted on line protector modules. In the first test, modules are placed in a packing carton, and the carton is dropped from a standard height, usually three feet. In the second test, a single module is dropped from a standard height, usually five feet, in such a way as to land on the grounding pins. Under these tests, the prior art springs could usually survive the packing carton test, however only the more expensive volute springs tended to pass the individual module test.
Another problem with some less expensive prior art springs is that they failed the individual module drop test, since they would become terminally or nonrecoverably compressed as a result of the drop. More particularly, these springs became compressed when absorbing the continued downward movement of the gas tubes and solder pellets upon impact with the bottom test surface. However, the momentum of the gas tubes and pellets compressed the springs beyond their limit and once so compressed they could not return to their free or original uncompressed length. This resulted in an unacceptably low holding force being thereafter exerted upon the gas tube and solder pellet and the frame member, and in many cases resulted in the gas tube and solder pellet becoming dislodged from the assembly. This result was unacceptable since when the gas tube and solder pellet are dislodged from the assembly they cease to serve their functional purpose in protecting conductor lines and equipment attached thereto from overvoltages.
Another problem with the prior art springs is that in an attempt to prevent terminal compression of the springs, the springs were strengthened and made less compressible. By making the spring too strongly compressive on the gas tube and solder pellet, the line protector was potentially subjected to a short circuit condition of the type which should only occur upon melting of the solder pellet in a long duration overvoltage (i.e., overcurrent) condition. Therefore, increasing the strength of the springs did not provide a solution to the problem of over-compression (i.e., nonrecoverable compression) resulting from the drop tests.